Prior to the latter 5th century the overlordship of Leinster was held by
the Fir Domnann. They are sometimes cited as a tribe of Firbolgs, usually called Damnonii.
The Fir Domnann were claimed to be connected to the
Dumnonii tribe who invaded Leinster sometime before the 4th century, and
were said to have come from western Caernarvonshire, south of Anglesey, in
Wales. Another very early conquering tribe comes down from native
tradition as the Gáileóin, who perhaps can be later identifed as the
Gailenga of Meath and north county Dublin. Galion and Domnand, alias Laigin, as said in Táin bó Cualnge.

The last of the Dumnonians ruled in the 5th century under the tribal name
of Dál Messin Corb. They were ousted by what may be called the
original Laigin tribes of the Uí Failge, Uí Bairrche and
Uí Enechglaiss. At around the same period the Loígis and
Fothairt were mercenary tribes of the Laigin and probably of
Cruithin (Pict) origin. The Uí Bairrche are in turn said to be related to
the Brigantes tribe of northern Britain, and they ruled southern Leinster
from the earliest centuries A.D. until their power was broken by the
Uí Cheinnselaig.

In the 9th century the chief dynasties which controlled all of the
southern and central regions of Leinster were the Uí Cheinnselaig and
the related tribes of Uí Dega and Uí Drona.

The northern regions were controlled by the Uí Dúnlainge with
their stronghold at Naas from the 7th century. In the 8th century the
Uí Dúnlainge dynasty branched out into three powerful septs of
Uí Muiredaig, Uí Dúnchada, and Uí Fáeláin. The family
name of the Uí Dúnchada at the time of the Norman Invasion was Mac Gilla
Mo-Cholmóg, which later became Fitz Dermot. Descendants of the Uí Muiredaig
included the O'Tooles. Descendants of the Uí Fáeláin included the O'Byrnes.
Click here for a Physical Map of the Leinster region.

Genealogy Lore of the Tribes of Laigin

The Laginian tribes of Leinster descend from Cú Corb, a descendant of Find File.
From Cú Corb's son Messcorb descend the Dál Messin Corb and the
Uí Garrchon (Ua Ferghaile or O'Farrelly), later to be referred to as the Fortuatha, or Alien Tribes.
From Cú Corb's son Cairpre descend the Dál Cairpre Arad, whose territory was in Munster.
From Cú Corb's son Corbmac descend the Dál Cormaic, Uí Gabla, Uí Labrada and Uí Buide (O'Kealy).
From Cú Corb's son Nia Corb descended Cathair Mór and Maine Mál.

Maine Mál was the ancestor of the Uí Máil, which included the septs
of the Uí Theig (O'Tighe) and Uí Ceallaig Cuallan (O'Kelly of the Wicklow hills).

Cathair Mór was the ancestor of the Free Tribes of Leinster: through his
son Ross Failge descended the Uí Failge. The Uí Failge dynasty later
divided into the three septs of Ua Conchobair Failghe (O'Connor Faly),
the Uí Riacáin (e.g. O'Dunne), and the Clann Máellugra (e.g. O'Dempsey). Through
Cathair Mór's son Crimthann descended the Uí Crimthainn Áin (e.g. O'Duff),
through Cathair's son Daire Barrach descended the Uí Bairrche (e.g. O'Gorman),
through Cathair's son Ailill Cétach descended the Uí Cheithig,
through Cathair's son Bressal Enechglass descended the Uí Enechglaiss
(O'Feary), and through Cathair's son Fiachu Baicced descended powerful
septs who were to later dominate Leinster prior to the Norman Invasion of
the late 12th century.

Bressal Bélach, son of Fiachu Baicced, was the father of Enna Nia and
Labraid Laidech. Enna Nia was the progenitor of the Uí Fergusa, the
Uí Briúin Cuallan (Cosgrave), and the Uí Dúnlainge. Labraid
Laidech was the ancestor of the Uí Dega (O'Hay), the Uí Cheinnselaig
and the Uí Dróna (O'Ryan).

The Uí Dúnlainge dynasty branched out into the three powerful septs of
Uí Muiredaig (O'Toole), Uí Dúnchada (Fitz Dermot), and
Uí Fáeláin (O'Byrne). They dominated northern Leinster in the
centuries prior to the Norman Invasion.

The Uí Cheinnselaig dynasty branched out into the powerful sept of
the Sil Fáelchán (Mac Murrough), as well as the septs of the
Uí Fergusa, the Uí Felmeda Thes (Murphy), the
Uí Felmeda Tuaid (O'Garvey), the Sil Chormaic, and the
Sil Máeluidir (Hartley). They dominated southern and central
Leinster in the centuries prior to the Norman Invasion.

Non-Laginian tribes of Leinster included the Loígis (O'More), the
Benntraige (O'Coskry), the Fotharta Fea (O'Nolan), and the
Fotharta in Chairn (O'Larkin).

Tribes outside the 750 A.D. Laigen territorial boundary (and within the
boundary of modern Leinster province) included the powerful
Southern Uí Néill septs of Clann Cholmaín and
Síl nÁedo Sláine of Mide and Brega respectively. The territory of Osraige (Co. Kilkenny and
southeast Co. Laois) was also not included in Laigen (Leinster) at this time, but instead was under the
authority of Munster (Mumu or Mumhan). Éle (or Ely) in southern Offaly extended further south into Co. Tipperary and was considered part of Munster (Urmuma or Ormond).
Surnames of the Province of Leinster